Add mld6 source file to fix missing include

[mkardous-silabs]

Problem / Feature

The mld6 ondemand timer feature required lwip timeout function but did not include the correct header causing implicit declarions.

Change overview

PR adds a copy / paste of the mdl6.c source file to allow CSA builds to work until the fixed file is released with the next patch.

PR also does a small clean up of files that exist from the initial purpose of the repo (lightweight GSDK).

Testing

Built the 917 ICD sleepy lock app.